1. Field of the Invention
The invention relates to a method for cleaning a polysilicon fraction.
2. Background Art
Highly pure polycrystalline silicon, referred to below as polysilicon, is produced on a large scale by thermal decomposition of a gas containing silicon or a gas mixture containing silicon in so-called Siemens reactors. The polysilicon is thereby created in the form of rods. These rods are subsequently comminuted with metal breaking tools such as jaw or ball crushers, hammers or chisels. This leads to superficial contamination of the polysilicon fragments. This contamination, and in particular any metal contamination, must be removed before the polysilicon fragments are processed further since they are unacceptable in the starting material for the production of electronic components and solar cells, for which the polysilicon fraction is generally used.
U.S. Pat. No. 6,309,467 B1 describes at length the prior art for cleaning polysilicon fragments and discloses a three-stage cleaning method for polysilicon, which subsequently has a very low iron and/or chromium content. With this method, the cleaning is carried out successively in a plurality of chemical treatment steps. Multistage cleaning methods require systems having a plurality of vessels, which are correspondingly elaborate. The method described in U.S. Pat. No. 6,309,467 B1 for cleaning a finely divided polysilicon fraction is disadvantageously also associated with a high consumption of acid and a high silicon loss. Furthermore, elaborate cooling is needed for cleaning a finely divided polysilicon fraction.